Our present invention relates to a metering system for a coating apparatus and, more particularly, to an apparatus for coating webs of a material such as paper, or cardboard. The invention, more particularly, relates to a metering system of the type in which a doctor bar is received in an elastic bed which is pressed toward a surface to be coated, i.e. the surface of the web as it is passed along a drum or roller surface, or the surface of a transfer roller from which the coating is applied to the web, and wherein the doctor bar is pressed toward that surface by a bladder, hose or tubular element extending along the width of the web and subdivided along the web into individually pressurizable compartments.
For the coating of paper or cardboard webs, the coating apparatus usually is provided with a coating drum or roller along which the web is guided and a doctor system for removing the material coated onto the web to the desired basis weight or thickness of that coating, or removing excess material from a transfer roller or drum which then applies the coating onto the web. The coating material in either case is applied in excess and the metering system can comprise a doctor element which wipes off the excess to the desired basis weight (coating weight per unit area of the coated surface).
The application of the flowable material and the metering, as noted, can be carried out directly on the web, preferably in a region in which the web is deflected, e.g. around a roller or drum, or on a transfer roller from which the belt is then applied to the web.
German patent document DE-A 30 22 955 describes a metering system in which the doctor element is a doctor bar held in a bed of an elastomeric material. The bed itself is held in a holder mounted on the machine frame and is pressed toward the surface to be coated by compressed air pressurization of a bladder or hose.
German patent document DE 43 41 341 C describes a metering system in which the bladder or pressurizable hose is subdivided into a plurality of individually pressurizable compartments which can be separately supplied with compressed air. In this metering system, the pressing forces generated by the compartmented bladder arrangement varies over the width of the web and the surface to be coated and creates nonuniform coatings in terms of the basis weight at various locations across the web. As a consequence, the basis width tolerance must be increased and control efforts made so that the nonuniformity does not detrimentally affect the product. In many cases it is not possible to vary the basis weight at different locations along the width to compensate for the irregularities which may arise in the earlier system. Furthermore, it is desirable to permit the basis weight to be adjusted differently at different locations, for example, to compensate for thickness variations in the paper web or to allow reduced basis weight coating at the edges of the web.
It is the principal object of the present invention to provide a dosing or metering system for a coating apparatus of the type described that permits a uniform basis weight to be applied to a web in a precise and delicate or sensitive manner.
Another object of the invention is to provide an improved coating system which is free from drawbacks of earlier systems.
These objects are attained, in accordance with the invention by providing at the back side of the doctor-bar bed, between the pressurizable bladder with its multiplicity of individually pressurizable compartments, and the bed, an elastic element which functions as a force-balancing cushion in the boundary regions between each two pressure compartments, thereby enabling the distribution of the force applied by the bladder along the bed in a more uniform manner.
More particularly, the metering system for an apparatus of the type described can comprise:
a doctor bar adapted to doctor a coating of a flowable material onto a surface and extending transversely to a path of a web to be coated and across a width thereof;
an elastic bed receiving the doctor bar and urging the doctor bar toward the surface;
a holder receiving the bed;
a pressurizable bladder braced against the bed upon pressurization with a fluid and subdivided across the width of the web and along the length of the bar into a plurality of individually pressurizable compartments, each separately supplied with compressed air; and
an elastic element extending along the bar and between the bladder and a side of the bed opposite that at which the bar is provided and forming a cushion balancing force applied by the bladder along the bed.
According to a feature of the invention, the modulus of elasticity of the elastic element is variable. The elastic element can include a continuous pressurizable hose or tube extending the full web width and along the full length of the doctor bar. The continuous tube can form one piece with the bed and the bed, the pressurizable tube and the bladder subdivided into individually pressurizable compartments can be received in a common transversely shiftable holder.
The pressurizable compartments can be provided between the back wall of the holder and the rear of the tubular elastic element and, if desired, a central rib which extends parallel to the doctor bar can connect the elastic element with the bed. The continuous pressurizable tube can have a variable pressing area or surface bearing on the back side of the bed. Each of the pressurizable compartments of the bladder can have a compressed air type which is guided out of the holder. The wall thickness of the continuous tube should be greater than the wall thickness of the bladder.